1. Field of the Invention
The present invention relates to an aqueous ink used in a system that a reaction liquid and an aqueous ink are used to form an image, a set of a reaction liquid and an aqueous ink, and an image forming method.
2. Related Background Art
Ink-jet recording is a printing method that minute droplets of an ink are ejected to apply them to a recording medium such as paper, thereby conducting printing, has a feature that high-resolution and high-quality images can be printed at high speed by a cheap apparatus and has been commonly used as means capable of simply forming high-quality color images in recent years. There is however a strong demand for formation of higher-quality color images.
On the other hand, various proposals have heretofore been made for forming higher-quality color images. For example, it has been proposed to use a black ink having characteristics of thickening or aggregating by an action with a salt and a color ink containing the salt in combination, thereby providing high-quality color images high in image density and free from occurrence of color bleed (bleeding due to color mixing between different colors) (see, for example, Japanese Patent Application Laid-Open No. H6-106735). In other words, according to this method, 2 inks different in properties from each other, i.e., the color ink containing the salt, which is a second liquid, and the black ink, which is a first liquid, are used to form a print, whereby components in these two liquids are caused to react to form aggregates of colorants, thereby permitting formation of good-quality images. Specifically, a salt of a polyvalent metal ion is used as the salt.
Various proposals have also been made as to formation of high-quality color images by using a set composed of a combination of a liquid composition containing no colorant and an ink, i.e., 2 liquids different in properties from each other (see, for example, Japanese Patent Application Laid-Open Nos. H9-207424, H11-78212, 2000-44855 and H9-286940).
The present inventors have carried out a further extensive investigation as to the ink sets containing the polyvalent metal salt among these proposals. As a result, it has been confirmed that printing is conducted by such a printing method as disclosed in Japanese Patent Application Laid-Open Nos. H6-106735 and H11-78212, whereby high-quality color images high in image density and free from occurrence of color bleed can be provided. However, this recording method involves a new problem because of too high reactivity.
More specifically, a reaction is almost completed at a point of time (from the moment of contact to several hundreds milliseconds) the reaction liquid has come into contact with the ink because the reactivity is too high. As a result, a color material component reacted with the reaction liquid is in a state remained on a surface-layer portion of the recording medium. It goes without saying that the colorant component may be removed in some cases when a printed portion is rubbed. Such a phenomenon possibly causes a trouble upon double-side printing.
In order to solve this problem, it has been confirmed that a substance for increasing binding property is added into an ink, whereby permeation of a colorant is controlled, and a film may be formed on the recording medium to have an effect on abrasion at the recorded portion as disclosed in Japanese Patent Application Laid-Open Nos. H9-207424, 2000-44855 and H9-286940.
On the other hand, it is important to eject an ink as stable droplets from a minute nozzle of an ink-jet recording head for stably forming high-quality ink-jet recorded images. Specifically, it is important that the ink is prevented from solidifying in orifices of the ink-jet recording head due to drying of the ink in the orifices in order to conduct stable ink-jet recording. Such an ink containing the substance having binding ability as described above involves a problem in this regard. More specifically, a low-hydrophilic resin emulsion or the like, which is the substance having binding ability, adheres to orifices or the like of the ink-jet recording head, whereby clogging may be caused at the orifices in some cases.